Accurate current control in AC motor drives is required for high-quality torque regulation of polyphase AC motors. Such control of current in each phase winding of the motor requires that the instantaneous current levels be accurately sensed. Presently available current controlled AC drives use discrete current sensors in series with the motor phase windings to sense the instantaneous phase current levels. These current sensors must be galvanically isolated from each other, as well as from the control logic, and must reject significant common mode signals associated with the inverter switching. Alternatively, AC drives can be constructed to include current sensors integral with the inverter switches and diodes. An example of this current sensor integration is described in T. M. Jahns et al. U.S. patent application Ser. No. 140,686, filed concurrently herewith and assigned to the assignee of the present invention. The integration of current sensors into the inverter switches and diodes facilitates reduction in size and cost of AC motor drives. In addition, other disadvantages associated with discrete current sensors including maximum operating temperature and sensor output drift are reduced with the new integrated sensors, as pointed out in the aforementioned Jahns et al. application Ser. No. 140,686.
However, a significant problem must be overcome when these integrated current sensors are used to regulate the output current in a direct current-to-alternating current (DC-to-AC) inverter circuit. FIG. 1a illustrates a typical inverter phase leg comprising two power switches in series connected across DC voltage source terminals 1 and 2, with the AC output power delivered at terminal 3. Each of the two switches 4 and 5 shown in FIG. 1a comprises an insulated gate bipolar transistor (IGBT) 6 and 7, respectively, combined with a flyback diode 8 and 9, respectively. As described in the aforementioned patent application Ser. No. 140,686, IGBT 6 and diode 8 are fabricated with integrated current sensors so that an output signal proportional to the bipolar switch current i.sub.1 is delivered at terminal 13. IGBT 7 and diode 9 are similarly equipped with integrated current sensors so that the associated switch current i.sub.2 can be measured at terminal 11.
It is desirable in such an inverter phase to regulate the instantaneous amplitude of the phase leg output current i.sub.3 which, according to Kirchoff's current law, is equal to the sum of i.sub.1 +(-i.sub.2). Thus, it becomes necessary to combine the sensor output signals at terminals 13 and 11 in order to obtain a measurement of the complete current i.sub.3 for current regulation feedback.
The difficulty in generating this measurement of current i.sub.3 arises because the current sensors in switches 4 and 5 do not share the same reference node. In particular, the reference node for the i.sub.1 measurement signal at terminal 13 is terminal 3, which switches between the positive and negative DC bus voltages at terminals 1 and 2 during normal operation. Since the reference node for the i.sub.2 measurement signal at terminal 11 is terminal 2, a large common-mode voltage difference exists between the i.sub.1 and i.sub.2 current measurement signals. This requires added instrumentation for level-shifting one of the signals to share the same reference voltage of the other sensor signal so that they can be combined to provide a single measurement of current i.sub.3 for current regulation purposes.
The necessity for coping with common-mode signals can be avoided by constructing an AC drive in which only the inverter phase-leg lower switches and diodes (i.e., those switches and diodes connected to the inverter negative DC input bus) contain integrated current sensors. Such a configuration, while avoiding the common-mode signal conditioning problem, results in current regulation being more difficult since complete feedback current data from the inverter are no longer available. Specifically, such feedback data are missing to measure current i.sub.1 when the inverter phase-leg upper switch-diode combination 4 conducts current. Thus the AC drive current controller must be able to accurately regulate motor phase currents by using current feedback information from the lower phase-leg switches and diodes only.